In particular, in premix burners the oxidizer air is provided by a fan while the gas flow rate is controlled by a valve.
The combustion zone may take several shapes. In general, the combustion head is provided with a cylindrical body which carries a covering element provided with a plurality of openings.
In particular, the covering element may be a metal fabric.
The mixture is ignited immediately after exiting from the openings of the premixing tube.
To date, most premix burners are characterised by a ratio equal to 5- to 10-fold the maximum power.
However, the spreading of the premix burner technology in increasingly diversified industrial applications, such as for example coating cabins, furnaces, drying plants etc. requires a wide range of the powers deliverable by the same burner. Moreover, in some cases, a considerable reduction of nitrogen oxide emissions is required at the same time.
In premix burners, such reduction is generally obtained by an increase in the air excess with respect to the amount of fuel used. However, such increase makes the lighting of the main flame quite difficult.
For completeness of description, FIG. 1 shows a known premix burner wherein the lighting of the main flame contemplates the use of a device 105 for lighting and maintaining a pilot flame.
The prior art shown in the above FIG. 1 contemplates the use of a premix burner 100 comprising a combustion head 110 and a plate 115 is integral thereto. Such plate 115 is used for attaching the combustion head 110 to the wall of a furnace, or of a drier (not shown).
An air flow is delivered to the combustion head 110 using dedicated fans (not shown), while the gaseous state fuel (combustible gas, or a vaporised liquid fuel) is made to flow towards the combustion head 110 using a duct (not shown).
Moreover, a certain amount of gaseous fuel is delivered to device 105 through a duct 116. The gas flow is regulated by a solenoid valve 117 controlled by electronic means of the known type and not shown. Exiting from duct 116, the combustible gas mixes with the air present in the combustion chamber (CC). Such mixture is lighted by a generating device 125. The pilot flame is then monitored for safety by a probe 130.
In turn, the combustion head 110 comprises a carrying body (not shown in FIG. 1) which supports a perforated covering element 120 provided with a plurality of openings 121.
The combustible/oxidizer mixture that is lighted in the combustion chamber (CC) by the pilot flame generated by the generating device 105 comes out through openings 121.
As shown in an enlargement of FIG. 1, the generating device 105 is attached to the combustion head 110 by means of a bracket 145 and comprises a screening shield 150 substantially perpendicular to the cylindrical wall of the same combustion head 110.
Moreover, for obvious safety reasons, the combustion of the fuel/oxidizer air mixture is externally triggered by the perforated covering element 120.
In order to work properly, an external pilot flame system of the type shown in FIG. 1 requires such pilot flame to be arranged in the vicinity of the combustion zone where it is subject to considerable thermal stresses during the routine operation of the burner. This implies the need of using special materials and screening elements.
Moreover, the delivery tube of the combustion gas to the generating device 105 is external to the combustion head and in general is in an overheated zone and in some cases this makes the use thereof hazardous.
Moreover, in some cases the occupied space required by the dimensions of the generating device 105 makes the use thereof difficult.